The present invention relates to a gas turbine having two-stage combustion type combustors of multi-can type and a control method of the gas turbine, and, more particularly, to a method of controlling combustion effected simultaneously in a plurality of unit combustors so as to avoid unstable combustion.
A conventional gas turbine having two-stage combustion type combustors is shown in FIG. 5. The combustor comprises a plurality of unit combustors. Each of the unit combustors has a first stage combustion section 28 for effecting diffusion combustion and a second stage combustion section 13 for effecting premixed combustion. One of features of this kind of gas turbine combustor is in that a fuel air ratio, that is, a ratio between fuel and air can be changed in a wide range from an operation at starting to a rated load operation.
The gas turbine having two-stage combustion type combustors can achieve low NOx combustion even in a wide range of fuel air ratio, which is achieved by combustion control in the first stage combustion section 28 and in the second stage combustion section 13. In the first stage combustion section 28, diffusion combustion is effected which is wide in working range from an operation at starting to an operation at rated load. That is, first, combustion is effected only in the first stage combustion section 28, the combustion then is changed to simultaneous combustion that is effected simultaneously in the first and second stage combustion sections 28 and 13, and thereafter the combustion is effected mainly in the second stage combustion section 13.
In the second stage combustion section 13, premix combustion is employed in which combustion air 29 and fuel 31 from a fuel nozzle 30 are premixed in swirl vanes 14 and then premixed fuel air is burnt downstream of the swirl vanes 14 to reduce NOx production even in a high load range.
Outlet air 25 from a compressor 24 is introduced into each of the unit combustors 12 and then flowed into a gas turbine section 27 as exhaust gas 26. Flow rate control of combustion air 29 for the second stage combustion section 13 is executed by an air flow control device 32 according to a fuel flow rate.
As for the diffusion combustion, since an amount of air necessary to effect stable combustion of injected fuel can be used and remaining air also can be used for dilution, NOx concentration is dull to fuel air ratio and the diffusion combustion can be effected in a wide range of fuel air ratio. However, even if much air exists in the surrounding, there is a limit to reduction of NOx production only by the diffusion combustion, so that the second stage combustion section 13 employing premix combustion is necessary.
FIG. 6 shows a relationship between fuel air ratio and NOx concentration or CO concentration in the second stage combustion section employing premix combustion. As shown in FIG. 6, there is a range 35 of fuel air ratio in which both NOx concentration 33 and CO concentration 34 are low. That is, when fuel air ratio is adjusted to be within the above-mentioned ratio 35, stable and low NOx combustion can be effected.
In case a premix combustion system shown at the swirl vanes 14 of FIG. 5 is employed in the second stage combustion section 13 to realize the combustion, fuel and air mix well, whereby a fuel air ratio can be lowered and the lowered fuel air ratio makes it possible to lower flame temperature and reduce NOx concentration.
However, combustion changes finely by a difference of a combustor itself, a change in temperature or humidity of combustion air, or change in calorific amount or components of fuel. In particular, in case a gas turbine is provided with a multi can type combustor consisting of a plurality of unit combustors, the combustion states of the unit combustors are different from one another, so that stable combustion may not be effected.
The concentration of NOx generated by combustion is sensitive to a ratio of fuel air supplied for the combustion, and a stable combustion range is narrow, so that it is necessary to finely control the fuel air ratio. Further, flow rate control of the combustion air 29 at time of premix combustion is executed by the air flow control device 32 according to a fuel flow rate, as mentioned above.
Here, conditions of mounting of the unit combustors on the gas turbine will be explained, referring to FIGS. 7, 8, 9. As shown in FIG. 7, the unit combustors 12 are mounted on a left end of the gas turbine and a exhaust portion 36 is at a right end. As is apparent from FIG. 8, a plurality of the unit combustors 12 are arranged on the peripheral portion of the gas turbine body at regular intervals in the peripheral direction. Each of the plurality of unit combustors 12 is provided with an air flow control device. The air flow control devices control simultaneously or individually all the unit combustors, however, combustion conditions are apt to be influenced by influence difference of the combustors themselves, etc to become unstable.
For the above-mentioned reasons, in some cases, even if a secondary fuel which is fuel for the second stage combustion section were injected into the second stage combustion section, fire could not be transmitted from the first stage combustion section to the second stage combustion section not to fire the secondary fuel, or the second stage combustion section was misfired. In such cases, the secondary fuel is exhausted without being burnt, so that the efficiency of combustion is lowered drastically. For preventing such a large decrease in the efficiency, there may be used measured results of temperature distribution, using thermocouples 37 mounted circumferentially on the exhaust portion 36, as shown in FIG. 9, for measuring exhaust gas temperature.
FIG. 10 shows a graph of measurement results of temperature distribution. A normal temperature distribution under which each unit combustor works normally is represented by a curve 38, while a temperature distribution at abnormal time under which some of the unit combustors are in unstable combustion is represented by a curve 39. Under the temperature distribution 39 at an abnormal time, a partial decrease in the exhaust gas temperature appears, so that judgement of abnormal conditions of the unit combustors may be executed monitoring the temperature distribution. A curve 40 represents a temperature distribution under which all the unit combustors are simultaneously abnormal.
A conventional method of controlling a gas turbine is disclosed in JP A 2-86927 (1990), in which the gas turbine is continuously worked even if a thermocouple as a monitoring device becomes out of order.